1. Field of the Invention
The present invention relates to a thermal recording head and more particularly to a thermal recording head wherein protective layers are eliminated from heat resistors used in the thermal recording head.
2. Description of the Related Art
Thin-film thermal recording heads are an important component for thermal recording and thermal transcribing in such recording devices as facsimile machines and printers. The basic structure of a conventional thermal recording head is shown in FIG. 1. A substrate 1 is provided to a ceramic substrate (not shown). To the substrate 1 is provided a 500 to 1,000 .ANG. thick heat resistor layer 2. A barrier layer 3 is provided to the heat resistor layer 2 so as not to cover the portion of the heat resistor layer 2 for heating heat-sensitive recording paper. A thin-film conductor 4 is formed on the barrier layer 3 a distance away from the heating portion of the resistor. The conductor 4, the portion of the barrier layer 3 not covered by the conductor 4, and the heating portion are covered by an anti-corrosion layer 5. The anti-corrosion layer 5 is covered with an anti-abrasion layer 6.
The substrate 1 is a glass layer several 10s of .mu.m thick that is sufficiently smooth to allow formation of the heat resistor layer 2 thereon. The substrate 1 must thermally insulate the heat resistor layer 2 from the ceramic substrate, so that as much of the thermal pulse generated by the heat resistor 7 as possible is transferred toward the anti-corrosion layer 5 and the anti-abrasion layer 6. The substrate 1 must also cool the heat resistor layer 2 between heat pulses by transferring heat away from the heat resistor layer 2.
The temperature of the heat resistor layer 2 rises from an original temperature to between 250.degree. to 300.degree. C. during each 2 ms pulse voltage. Its temperature must cool to the original temperature during the subsequent 20 ms or so inter-pulse interval. A heat resistor for a thermal recording head must be durable enough to repeat this harsh cycle 100 million times without its rate of change of resistance exceeding + or -10%. Heat resistor materials should have resistivity between 1,000 to 2,000 .mu..OMEGA.cm because the practical range for thin-film thickness is between 500 to 1,000 .ANG.. Only a few conventional materials, such as Ta.sub.2 N, TiOx, and B.sub.2 Hf, successfully meet these requirements. Because all of these materials oxidize when heated in air, and burn out as a result, the anti-oxidation layer 5 is indispensable in conventional thermal recording heads as a layer for blocking oxygen from contacting the heat resistor layer 2. The anti-corrosion film 5 is generally a 3 to 5 .mu.m layer of SiO.sub.2 formed by sputtering. However, because the SiO.sub.2 layer is easily abraded by contact with the heat-sensitive recording paper, its surface must be covered with the anti-abrasion layer 6. The anti-abrasion layer 6 is usually a 2 to 3 .mu.m layer of Ta.sub.2 O.sub.5 formed by sputtering. The anti-oxidation layer 5 and anti-abrasion layer 6 also protect the thin-film conductor 4, which is usually formed from a soft metal such as aluminum, from abrasion.
If the thin-film conductor 4 were formed directly on the thin-film heat resistor layer 2, applying a voltage to the thin-film conductor 4 would generate electromigration in the heat resistor layer 2. Such electromigration greatly changes the resistance of the heat resistor layer 2. The barrier layer 3 insulates the heat resistor layer 2 from the conductor 4, thereby preventing electromigration. The barrier layer 3 is a thin-film layer, about 500 to 1,000 .ANG. thick, formed from a material with a high melting point, such as chromium.
The metal conductor 4 is 1 to 2 .mu.m thick to reduce its resistance. This thickness raises the surface level of the conductor 4 above that of the heating portion, creating a "hill and valley" situation, with the heating portion in the valley. The conductor 4 is usually formed at a position about 200 to 300 .mu.m away from the heating portion so the heat-resistant recording paper can contact the heating portion without being obstructed by the conductor 4. Positioning the conductor 4 a distance from the heating portion also minimizes heat loss to the conductor 4 which conducts heat better than the protective layers. Separating the conductor 4 and the heating portion by this distance allows lowering the resistance of the barrier layer 3 to about 1% that of the thin-film resistor layer 2. Heat loss can thus be suppressed.
Research fueled by the continuing demand for faster printing speeds has produced thermal printers, including thermal recording heads formed as described above, which can print with 1 ms heat pulse at frequencies of 100 Hz. However, to attain such high speeds, the heat resistor must be heated to high temperatures that create great thermal and mechanical distortion in nearby components. The warping can cause cracks in the anti-corrosion layer 5 and the anti-abrasion layer 6. These cracks can allow air to contact the thin-film resistor layer 2 which can burn out as a result.
High-speed facsimile machines and other products have been produced with thin-film heat resistors formed from oxidized materials, that is, materials stabilized by heat processes performed in air. For example, Japanese Patent Application Kokai No. SHO-58-84401 describes a thin-film heat resistor made from a Cr--Si--SiO alloy material and Japanese Patent Application Kokai No. SHO-57-61582 describes a thin-film heat resistor made from a Ta--Si--SiO alloy. These materials are extremely stable when heated in an oxidation atmosphere as long as the temperature is equal to or less than that of the heat processes.
An LSI circuit provided to thermal recording heads for energizing the heat resistor 7 with a voltage pulse is conventionally connected to the heat resistor 7 as shown in FIG. 2. A wiring substrate 8 is mounted adjacent to the substrate 1 on a heatsink 9. To the end of the wiring substrate 8 opposing the substrate 1 is connected a connector 10. The heat resistor 7 is mounted on the substrate 1. A drive LSI circuit 11 is connected to the wiring substrate 8 by a gold wire 12 and to the substrate 1 by a gold wire 12'. A resin 13 covers the gold wires 12 and 12' and the drive LSI circuit 11 for protection.
A problem has been known with commercially produced high-speed thermal recorders with the basic structure shown in FIG. 1 in that the anti-oxidation layer 5 and the anti-abrasion layer 6, totaling 5 to 8 .mu.m, prevent the heating portion of the heat resistor layer 2 from contacting the heat-sensitive recording paper directly. Also, almost half of the energy required for recording with conventional thermal recording heads serves to heat the protective layers instead of the heat-sensitive paper. Furthermore, the protective layers thermally buffer the heat-sensitive recording paper from the heat resistor layer, creating a delay from when the heat resistor heats to when the surface of the protective layers contacting the heat-sensitive recording paper heats. Further a great deal of heat that the heat resistor generates escapes to the substrate because of the undesirable thermal insulating properties of the protective layers.
There has also been known a problem with the method of connecting the LSI circuit 11 with the heat resistor 7 shown in FIG. 2 in that more connections by gold wires 12 and 12' are required than the number of heat resistors 7. Because so many gold wire connections are required, the cost of the gold wire accounts for one third the entire cost to produce the thermal recording head. This configuration also limits further decreases in size of the thermal recording head.